Thermoelectric properties of the solid solutions based on ThSi2-type CeSi2 compound

Article abstract of DOI:10.1016/j.jallcom.2005.07.053

Thermoelectric properties (from 240 up to 380 K) of the ThSi₂-type CeSi₂, Ce₄₀Si₂₇Ge₃₂, Ce₃₈Si₄₈Ge₁₃, Ce₃₇Si₂₈Ga₃₅ and Y_0.5

Full text of DOI:10.1016/j.jallcom.2005.07.053

Journal of Alloys and Compounds 415 (2006) 12–15
Thermoelectric properties of the solid solutions based
on ThSi2-type CeSi2 compound
A.V. Morozkina,∗, V.A. Stupnikova, V.N. Nikiforovb,
Nobuyoshi Imaokac, Isao Morimotoc
a
Department of Chemistry, Moscow State University, Leninskie Gory, House 1, Building 3, GSP-2,
Moscow 119992, Russian Federation
Physics Department, Moscow State University, Leninskie Gory, GSP-2,
b
Moscow 119899, Russin Federation
Asahi Kasei Corp. 2-1, Samejima, Fuji-city, Shizuoka 416-8501, Japan
c
Received 6 May 2005; received in revised form 22 July 2005; accepted 26 July 2005
Available online 29 September 2005
Abstract
Thermoelectric properties (from 240 up to 380 K) of the ThSi2-type CeSi2, Ce40Si27Ge32, Ce38Si48Ge13, Ce37Si28Ga35 and Y0.5Ce0.5Si2
compounds are reported.
The Seebeck coefficients increase with increasing temperature from 240 up to 380 K for the CeSi2 (S = −62 to −43 ␮V/K), Ce38Si48Ge13
(S = −25 to −13 ␮V/K), Ce40Si27Ge32 (S = −7.5 to +2 ␮V/K) and Y0.5Ce0.5Si2 (S = −25 to −14 ␮V/K) compounds. The Seebeck coefficient
desreases with increasing temperature for the Ce37Si28Ga35 compound (S = −8.8 to −13.6 ␮V/K). The substitution of Ge, Ga or Y for Si or
Ce shifts the Seebeck coefficient to positive values. The presence of the Si (Ge) impurity phase strongly influences the electrical resistance
and the thermal conductivity of the alloys.
The CeSi2 has the best ZT parameter of the present compounds: ZT = 0.086 at 240 K.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Rare earth compounds; Thermoelectric materials
1. Introduction
2. Experimental details
It is known that the RSi2 and RGe2 compounds (R = rare
earths) adopt the low-temperature GdSi2-type structure and
thehigh-temperatureThSi2-typestructure[1,2]. IntheThSi2-
type structure (space group I41/amd, no. 141-2) the rare earth
atoms occupy the special position 4(a) (0, 3/4, 1/8), the p-
element atoms occupy the 8(c) site (0, 1/4, ZX) [2]. RGa2 are
known to be AlB2-type compounds [1].
This paper reports on the crystal structure and thermo-
electric properties of solid solutions based on the ThSi2-type
CeSi2 compound.
In the present investigation, the compounds were pre-
pared in an electric arc-furnace under an argon atmosphere
using a non-consumable tungsten electrode and a water-
cooled copper tray. Germanium, silicon, gallium (purity all
components 99.99%), cerium and yttrium (purity compo-
nents 99.9%) were used as the starting components. Zir-
conium was used as a getter during melting. The samples
were remelted several times to obtain full homogeniza-
tion.
Subsequently, the CeSi2 and Y0.5Ce0.5Si2 alloys were
milled, pressed and subjected to a thermal-pressing treat-
ment (temperature 1470 K, pressure 5 GPa, time of treat-
ment 240 s). For this thermal-pressing treatment we used a
hydraulic press, DO-1386 (Riazan’ Industry plant, Russia)
∗
Corresponding author.
E-mail address: morozkin@general.chem.msu.ru (A.V. Morozkin).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.07.053

A.V. Morozkin et al. / Journal of Alloys and Compounds 415 (2006) 12–15
13
Table 2
and a standard high-pressure container “chechevitsa 028”
(Il’ich plant, St. Peterburg, Russia).
Interatomic distances D 2 × 10−3 nm and coordination number N for
atoms in the CeSi2 and Ce37Si28Ga35 compounds
The quality of the samples before physical measurements
was determined using X-ray phase analysis and microprobe
X-ray analysis. X-ray data were obtained on a DRON-3.0
diffractometer (Cu K␣ radiation, 2Θ = 20–70◦, step 0.05◦, for
5 s per step). The diffractograms obtained were identified by
means of calculated patterns using the Rietan-program [3,4]
in the isotropic approximation. A «Camebax» microanalyser
was employed to perform microprobe X-ray spectral analyses
and to obtain S.E.M. images of the samples.
(a) CeSi2
Atoms
(b) Ce37Si28Ga35
D
N
Atoms
D
N
Ce
Ce
4Si
8Si
4Ce
Si
0.315
0.316
0.4067
16
4Xa
8X
4Ce
X
0.322
0.321
0.4165
1Si
2Si
2Ce
4Ce
a
0.226
0.242
0.315
0.316
9
1X
2X
2Ce
4Ce
0.233
0.246
0.322
0.321
Theelecricalresistance, thermalconductivityandSeebeck
coefficient were measured with using standard methods [5]
on the laboratory equipment.
X = Si0.4Ga0.6
.
Thermal-pressing treatment of the GdSi2-type Y0.5Ce0.5
-
3. Results and discussion
Si2 compound leads to the formation of the high-temperature
ThSi2-type structure with increasing unit cell volume.
X-ray phase analysis and microprobe X-ray analysis show
that the samples contain an admixture of phases that may
influence the results of the measurements of the thermoelec-
tric properties (Table 3 and Fig. 1).
The temperature dependences of the Seebeck coefficients,
electrical resistance and thermal conductivity are given in
Fig. 2.
The CeSi2 and CeSiGa alloys show metallic type conduc-
tivitywithclosevaluesoftheelectricalresistance(thethermal
coefficients of the electrical resistance are 1.72 × 10−3 K−1
for CeSi2 and 1.43 × 10−3 K−1 for CeSiGa) (Fig. 2b). The
CeSiGe and Y0.5Ce0.5Si2 alloys show semi-metallic type
conductivity (the thermal coefficients of the electrical resis-
tance are 7.26 × 10−5 K−1 for CeSiGe and 2.18 × 10−4 K−1
for Y0.5Ce0.5Si2 alloys). The CeSi1.6Ge0.4 alloy shows semi-
conductor type conductivity (Fig. 2b). It is obvious, the
presence of the Si(Ge) phase strongly influences the electri-
cal resistance of the CeSiGe, CeSi1.6Ge0.4 and Y0.5Ce0.5Si2
alloys (Fig. 1; Table 3).
Analysis of the powder X-ray diffractograms shows that
the solid solutions based on the CeSi2 compound and pre-
pared by arc-furnace melting crystallize in the tetrago-
nal ThSi2-type structure (space group I41/amd, no. 141-2),
excepting Y0.5Ce0.5Si2. The Y0.5Ce0.5Si2 compound crys-
tallizes in the orthorhombic GdSi2-type structure (space
group Imma, no. 74). The cell parameters of the compounds,
refined at room temperature, atomic position papameters
and the reliability factor RF are given in Table 1. The
interatomic distances for CeSi2 and Ce40Si27Ge32 com-
pounds are given in Table 2. The substitution of germanium
or gallium for silicon leads to increasing cell parameters
in the CeSi2 compound. The substitution Y for Ce leads
to decreasing the cell parameters in the CeSi2 compound
(Table 1).
The thermal-pressing treatment leads to a decreasing unit
cell volume and an anisotropic distortion of the unit cell in
the CeSi2 compound (the a cell parameter decreases, whereas
the c cell parameter increases) (Table 1). After arc-furnace
melting the CeSi2 compound has c/a = 3.3204, after thermal-
pressing treatment it has c/a = 3.3478. Probably, the result
of this thermal-pressing treatment is a decreasing number of
defects in the polycrystalline lattice of the alloys.
Thethermalconductivityforallcompoundsincreaseswith
increasing temperature (Fig. 2c). The CeSiGe alloy shows
lowest value of the thermal conductivity. The thermal con-
ductivities of the other alloys are close.
Table 1
Cell parameters a (nm), b (nm) and c (nm), unit cell volume V (nm3) and atomic position parameters of RX2 compounds
Compound
Structure
Space group
a
b
c
V
ZX
RF
CeSi2
CeSi2
ThSi2
ThSi2
GdSi2
ThSi2
ThSi2
ThSi2
ThSi2
I41/amd
I41/amd
Imma
0.41874(4)
0.41711(5)
0.4154(6)
0.41219(4)
0.4229(1)
0.42018(4)
0.4243(1)
1.3904(2)
1.3964(2)
1.3491(3)
1.3554(1)
1.4353(2)
1.3865(2)
1.3897(3)
0.24380
0.24295
0.22852
0.23028
0.25670
0.24479
0.25019
0.457(1)
0.456(1)
7.5
8.0
8.7
4.6
5.7
7.6
6.9
a
b
a
Y
0.5Ce0.5Si2
0.40776(6)
Y0.5Ce0.5Si2
I41/amd
I41/amd
I41/amd
I41/amd
ꢀ
0.4570(6)
0.4561(6)
0.458(1)
0.457(1)
Ce37Si28Ga35
Ce38Si48Ge13
Ce40Si27Ge32
ꢀ
The R factors are given in percent (RF = 100(
|(Ikobs
)
1/2 − (Ikcal
)
1/2|)/
|(Ikobs
)
1/2|)%, Ikobs is the integrated intensity evaluated from summation of
k
k
contribution of the kth peaks to net observed intensity, Ikcal is the integrated intensity calculated from refined structural parameters).
a
After thermal-pressure treatment.
b
Atomic position parameters: rare earths atoms occupy 4(c) special position [0,1/4, 375(1)], silicon atoms occupy 4(c) site [0,1/4, 0.778(3)] and 4(c) site
[0,1/4, 0.947(3)].

14
A.V. Morozkin et al. / Journal of Alloys and Compounds 415 (2006) 12–15
Table 3
Mass fraction, composition and crystallographic data of the phases in the RX2 alloys
Alloys
Phasea
Mass fraction
Type structure
Space group
a (nm)
b (nm)
c (nm)
RF (%)
CeSi2
CeSi2
Y0.5Ce0.5Si2
Ce33Ge67
Ce33Si67
1.00
1.00
0.94 0.06
ThSi2
ThSi2
GdSi2 C
I41/amd
I41/amd
Imma Fd3m
0.41874(4)
0.41711(5)
0.4154(6)
0.5407(6)
0.41219(4)
0.5418(5)
0.4229(1)
0.8277(5)
0.42018(4)
0.4243(1)
1.3904(2)
1.3964(2)
1.3491(3)
7.5
8.0
8.7 9.0
b
Ce18
Y
16Si66 Si99
0.40776(6)
b
c
Y
0.5Ce0.5Si2
Ce18Y16Si66 Si99
0.95 0.05
0.92 0.08
ThSi2 C
I41/amd Fd3m
I41/amd Pnma
1.3554(1)
4.6 6.6
5.7 5.0
CeSiGa
Ce37Si28Ga35
Ce50Si39Ga11
Ce38Si48Ge13 Si100
Ce40Si27Ge32
Si70Ge30
ThSi2 FeB
0.3984(2)
1.4353(2)
0.5972(4)
1.3865(2)
1.3897(3)
CeSi1.6Ge0.4
CeSiGec
∼0.99 ∼0.01
∼0.99 ∼0.01
ThSi2 C
ThSi2 C
I41/amd Fd3m
I41/amd Fd3m
7.6
6.9
ꢀ
ꢀ
The R factors are given in percent (RF = 100(
|(Ikobs
)
1/2 − (Ikcal
)
1/2|)/
|(Ikobs
)
1/2|)%, Ikobs is the integrated intensity evaluated from summation of
k
k
contribution of the kth peaks to net observed intensity and Ikcal is the integrated intensity calculated from refined structural parameters).
a
Data of microprobe X-ray analysis.
After thermal-pressure treatment.
b
c
The C-type phase was detected for the microprobe X-ray analysis. For CeSi1.6Ge0.4 and CeSiGe alloys the mass fraction has approximately value.
The Wiedemann–Franz parameter strongly differs from 1
for the CeSiGe, CeSi1.6Ge0.4 and Y0.5Ce0.5Si2 alloys (Fig. 3).
The Wiedemann–Franz parameter is WF = ρ·K/γVF·T. Here,
ρ is the electrical resistance, K the thermal conductivity, γWF
the Wiedemann–Franz constant γWF = (π2/3)·(kB/e)2, kB the
Boltzmann constant and e the electronic charge.
Of course, the lattice thermal conductivity Klattice of these
alloys may strongly influence the general thermal conductiv-
ity of the compound (the thermal conductivity K is the sum of
the lattice conductivity Klattice and the electronic conductiv-
ity Kelectronic: K = Klattice + Kelectronic). However, the presence
of the Si(Ge) phase in these alloys may be the main reason
for such behaviour of the electrical resistance and thermal
conductivity.
The Seebeck coefficient of all compounds increases with
increasing temperature, excepting the CeSiGa alloy (Fig. 2a).
That is why the ZT parameter decreases with increasing tem-
perature for all alloys, excepting CeSiGa (Fig. 2d and e). The
Fig. 1. SEM images of the CeSi2 (a), Y0.5Ce0.5Si2 (b) alloys after the thermal-pressing treatment and CeSi1.6Ge0.4 (c) and CeSiGe (d) alloys.

A.V. Morozkin et al. / Journal of Alloys and Compounds 415 (2006) 12–15
15
Fig. 3. The Wiedemann–Franz (WF) parameter vs. temperature for ThSi2-
type compounds.
ZT parameter has a maximal value of 0.086 at 240 K for the
CeSi2 compound. All substitutions of Ge, Ga or Y for Si or
Ce in the CeSi2 compound shift the Seebeck coefficient to
less negative values.
4. Conclusion
1. The presence of the Si(Ge) impurity phase strongly influ-
ences the electronic transport properties of the investi-
gated alloys.
2. The CeSi2 compound has a maximal value of ZT = 0.086
at 240 K, whereas the ZT parameters of the other alloys
based on CeSi2 are lower.
3. All substitutions of Ge, Ga or Y for Si or Ce in the CeSi2
compound shift the Seebeck coefficient to less negative
values.
Acknowledgement
This work is supported by Asahi Kasei Corporation
(Japan) in the ISTC project N 2382p.
References
[1] E.I. Gladyshevsky, O.I. Bodak, Kristallohimia intermetallicheskih
soedinenii redkozemel’nyh metallov, L’viv, LSU 144 (1982) (in Rus-
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[2] Pearson’s Handbook of Crystallographic Data for Intermetallic Phases,
American Society for Metals, Metals Park, OH 44073 (2–3) 1985.
[3] F. Izumi, RIGAKU J. 6 (1) (1989) 10–19.
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tivity (K) and ZT parameter vs. temperature for CeSi2, CeSiGa, CeSiGe,
CeSi1.6Ge0.4 and Y0.5Ce0.5Si2 compounds.
[4] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford Uni-
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